Distribution and Main Controlling Factors of CO2 in Santos Basin, Brazil
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摘要: 桑托斯盆地盐下油气田中发现了大量CO2,给油气勘探开发和生产都带来诸多困难和挑战.利用地层测试、样品分析及文献资料等,明确了CO2成因及来源,统计分析了其区域分布特征,并基于区域重磁和深源地震等资料,剖析了控制CO2分布的地质因素.盆内CO2主要为幔源—岩浆成因,且幔源CO2贡献了至少92%的CO2总量.区域上,CO2自陆向海呈增加趋势,并相对集中在盆地东部隆起带上.地壳减薄和地幔局部隆升是控制CO2宏观分布最重要的背景因素.极端的地壳伸展造成了圣保罗地台下部陆壳强烈拉伸减薄,形成了一个面积约5.1×104 km2的地壳减薄区,造成了富含CO2的地幔物质上拱进入陆壳,宏观上决定了盆内CO2区域分布.此区域之外,出现高含量CO2的可能性大幅降低.岩浆侵入和活动断层都是沟通隆升地幔和浅部储层的重要路径,但以断裂沟通最常见.NW-SE向区域走滑断裂和NE-SW向I-II级正断层对CO2在浅部地层中的分配起控制作用,两组断裂交汇部位或周缘是幔源岩浆或CO2最集中发育区.Abstract: An extraordinarily high amount of CO2found in pre-salt section in Santos basin poses great challenges to the oil and gas exploration and development. In this study, combining regional gravity and magnetic database, deep-seismic lines, well drilling, formation test and samples dataset of the basin, the origin of CO2 was clarified firstly, and its regional distribution patterns were statistically analyzed and their main controlling factors were explored as well. The widely distributed CO2 in pre-salt reservoirs in Santos basin is mainly sourced from mantle, and mantle-derived CO2 contributes at least 92% of the total volume of CO2. Regionally, CO2 abundance gradually increases from continent to ocean, and is relatively concentrated in the eastern uplift of the basin. On its margin usually occurs the current maximum CO2 content values. The extra-high CO2 abundance in Santos basin is the result of the combined action of various geological factors such as crust & mantle transition, regional tectonic evolution, magma events and fault activity. Among them, continental crust thinning and mantle uplifting are the most important background factors, controlling the regional distribution of CO2. The intensive extension of crust caused a strong stretching and thinning of the lower continental crust of Sao Paulo platform, forming a crustal thinning area, around 5.1×104 km2, which caused the CO2-rich mantle material to intrude upward into the continental crust. This uplifted mantle has directly controlled the regional distribution of CO2 in Santos basin. Outside this area, the risk of CO2 is significantly reduced. Magma injection or active faults are both important pathways for CO2 migration and accumulation, with most common cases involving active faults leading CO2 from "uplifted" mantle to shallower reservoirs. The NW-SE strike-slip faults and the NE-SW class I-II normal faults have an obvious control effect on distribution of CO2 in shallower formation: NW-SE strike-slip fault could extend into deep mantle, while the NE-SW normal faults distribute these CO2 in shallower layers. So that their intersection points or periphery areas are the most favorable areas for accumulation of magma and CO2.
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图 1 桑托斯盆地构造单元划分及综合地层柱状简图
图a据Sandwell and Smith(2009)编绘; 走滑断裂体系据Cobbold et al.(2001)、Meisling et al.(2001)和Evain et al.(2015)编绘; 地层柱状图据Moreira et al.(2007)编绘
Fig. 1. Structural unit division and integrated stratum column of Santos basin
图 2 桑托斯盆地CO2同位素特征及幔源CO2含量估算
a. CO2成因判识图,据Dai et al.(1996);b.幔源CO2含量估算图,据Santos et al.(2012)
Fig. 2. Carbon isotope of CO2 and mantle derived CO2 content estimation
图 3 桑托斯盆地CO2分布及盆地深部地壳结构简图
区块分布据http://rodadas.anp.gov.br/arquivos/Bienio/Mapas_LP7-8/R7_R8_LP_GERAL.pdf
Fig. 3. Sketch map of CO2 distribution and deep crustal structure in Santos basin
图 4 Jp构造及周缘预测岩浆体与CO2分布对应关系
据Gamboa et al.(2019)改编;a. 磁力图一阶导数;b. 剩余布格重力图;c. 二维地质解释模型(位置见图a和b中黄色箭头)
Fig. 4. Distribution maps of predicted igneous rocks and CO2 around the Jp structure
图 5 桑托斯盆地深源地震剖面及深部壳幔结构剖面
剖面位置见图 3;图 5a和5d中二维地震剖面底图据Kumar et al.(2012)修编;图 5b和5c中San Ba地震剖面底图据Evain et al.(2015)修编,其中白色虚线为Zalán et al.(2011)解释出的基底构造和莫霍顶面
Fig. 5. Deep crustal structure profiles from deep-source seismic data in Santos basin
图 6 桑托斯盆地Lb区块断裂与CO2分布
区块位置见图 3;图a据Rancan et al.(2018)和Zhao et al.(2019);图b为地震速度剖面;图c和d为常规地震剖面,其中c未带解释方案剖面,d为带解释方案剖面;黄色箭头指CO2,黑色箭头为岩浆
Fig. 6. Distribution map of faults and CO2 in Lb block
表 1 Lb区块不同构造位置CO2含量及流体性质统计
Table 1. Statistic table of CO2 content and fluid properties in Lb block
样品位置 取样位置 样品类型 闪蒸气体中CO2(摩尔百分比) 原位地层中CO2(摩尔百分比) 气油比(m3/m3) 原油密度(°API) 样点1 ① 气 67.16 65.26 3 077.17 36.36 ② 气 67.55 65.37 2 834.43 37.67 ③ 气 67.58 65.51 2 954.30 37.25 ④ 气 67.32 65.31 2 958.99 37.01 ⑤ 气 66.95 64.88 2 931.61 37.57 样点2 ⑥ 油 43.40 37.29 436.13 27.28 ⑦ 油 43.37 37.25 438.64 27.15 注:取样位置见图 6c. -
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